Light-emitting devices

ABSTRACT

A light-emitting device comprising a first electroluminescent element for emitting light of a first colour when energised and a second electroluminescent element for emitting light of a second colour when energised, characterised in that dimensions of the first and second elements and any spacing therebetween are selected to create an overall impression of a single light source when simultaneously energised and viewed as intended.

DESCRIPTION

[0001] The present invention relates to a light-emitting device,particularly but not exclusively, an electro luminescent device for useas a backlight to an LCD display, and displays incorporating suchdevices. The electroluminescence for the electroluminescent device maybe suitably provided by means of an organic light-emissive material (seefor example International Publication WO90/13148).

[0002] By way of background, FIG. 1 shows the typical cross-sectionalstructure of an organic light-emissive device. The device is fabricatedon a substrate (1) coated with a transparent first electrode (2) such asindium-tin-oxide. The coated substrate is overcoated with at least onelayer of a thin film of an electroluminescent organic material (3) and afinal layer forming a second electrode (4) which is typically of metal.By using a transparent substrate (e.g. of glass or plastics material),light generated in the film (3) is able to leave the device by passingthrough the first electrode (2).

[0003] The performance of electroluminescent devices has advancedrapidly over the past few years. Due to their high efficiencies, thedevices show potential for a wide range of display applications, fromsimple backlights to graphic displays, such as television screens,computer monitors and palm-top devices which may consist of severalmillion pixels. In particular, organic light-emissive material may beused in the fabrication of backlights for transmissive or transflectiveliquid crystal displays. In a liquid crystal display there is typicallya planar liquid crystal cell which has active regions where the opticalproperties of the liquid crystal material can be altered by theapplication of an electric field to vary the transmission of lightthrough the active regions. In a transmissive liquid crystal displaythere is a light source behind the liquid crystal cell; and light fromthe source shines to an observer through those regions permitted totransmit light. In a transflective liquid display, the light source issupplemented by a reflective mirror, also behind the liquid crystalcell, which can return incident light towards the observer.

[0004] Organic light-emissive materials have been devised to emit lightover a range of colours, and a full set of red- green- and blue-emittingpolymers are now available. Emission colour of electroluminescentmaterials in devices has been controlled in one of several ways.Firstly, it is known to use a semiconductive conjugated co-polymercomprising at least two chemically different monomer units which havedifferent semiconductor band gaps when existing in their individualhomo-polymer forms. The relative proportions of the monomer units may bevaried to alter the semiconductor band gap so as to control the opticalproperties of the resultant co-polymer (see WO92/03490). Anotherapproach, particularly suitable for producing a single whitelight-emitting layer, involves depositing on a substrate a mixture ofblue and red-type organic electroluminescent materials using a flashvacuum deposition process. (See JP 0921989).

[0005] The present applicant has appreciated the desirability of whitelight-emitting devices, both for use as point sources as well asbacklights for LCD displays of mobile phones and the like. The presentapplicant has devised a novel device which is able to operate at a lowervoltage than white light-emitting, mixed organic electroluminescentmaterial known in the prior art.

[0006] In accordance with a first aspect of the present invention, thereis provided a light-emitting device comprising a firstelectroluminescent element for emitting light of a first colour whenenergised and a second electro luminescent element for emitting light ofa second colour when energised, characterised in that dimensions of thefirst and second elements and any spacing therebetween are selected tocreate an impression of a single light source when the elements aresimultaneously energised and viewed as intended.

[0007] The present applicant believes that such a light-emitting devicemay provide a convenient way of achieving a particular lighting effect,especially if the desired colour of the light is difficult to achievewith a single electroluminescent material. This is because in perceivinga single light source, it is believed the viewer viewing the device inthe intended manner will also perceive a single colour of light,determined in part by the sum of the first and second colours. If thefirst and second colours are different, the single colour of lightperceived will also be different.

[0008] The first and second elements may be energised by a common bias.In other words, the elements may share the same anode and cathode. Sucha construction would be relatively simple to manufacture, and is to becontrasted with known devices where different light emitting regions areindependently energised.

[0009] In one embodiment, at least one of the electro luminescentelements comprises an organic light-emissive material. The material maybe a polymer, perhaps a conjugated polymer. The organic light-emissivematerial may be deposited on a substrate, perhaps by a process ofink-jet deposition. Ink-jet deposition enables efficient, fine andaccurate definition of the at least one electroluminescent element. Eachelement may have a generally circular profile over the surface of thesubstrate. Ink-jet deposition could readily deposit such “dots” in acompact and reproducable array (65,000 dots per cm²).

[0010] The first electroluminescent element may be one of a plurality ofsuch elements for emitting light of the first colour when energised, theplurality of elements being arranged in a first spaced-apart array. Thesecond electroluminescent element may be one of a plurality of suchelements for emitting light of the second colour when energised, theplurality of second light-emitting elements being arranged in a secondspaced-apart array. The first and second spaced-apart arrays mayoverlap, with each element of the first spaced-apart array adjacent toelements of the second spaced-apart array. The first and secondspaced-apart arrays may share energising electrodes.

[0011] The dimensions of the discrete elements and any spacings betweenadjacent elements may be less than the limit of resolution of the nakedeye, say less than 1.0×10⁻⁴ meters when the object is placed at the nearpoint. The maximum dimension of the elements on the surface of thesubstrate may be less than 5.0×10⁻⁶ meters; and the maximum spacingbetween adjacent elements in the plane of the substrate may be less than5.0×10⁻⁶ meters. In one form, the spacing between adjacent elements maybe negligible or even non-existent (i.e. adjacent elements contact acommon insulator). Such fine dimensions and spacings may be particularlyuseful in small (hand held) electronic articles incorporating thelight-emitting device, where visual acuity and achieving maximum fillfactor (minimising dark areas) are important considerations. However,with larger articles, which are intended to be viewed from distancesgreater than one arm's length, the size of and spacings between elementsbecome less critical. This is because the minimum feature sizeresolvable to the naked eye is dependent on viewing distance (i.edistance between object and observer). The angular resolving power ofthe naked eye remains constant, so as the distance increases, so doesthe size of the minimum feature which is resolvable.

[0012] The first and second colours of the first and second organiclight-emissive materials may be selected from the group consisting ofred, green and blue. By selecting one red emitter and one green emitter,it may be possible to give the impression of a substantially whitelight-emitting device. The device may further comprise a thirdelectroluminescent element for emitting light of a third colour, thedimensions of the third element and any spacing between the third andadjacent elements being selected to create the impression of a singlelight source where all the elements are energised and viewed asintended. The first, second and third organic light-emissive materialsmay be selected to produce red, green and blue light emissionsrespectively.

[0013] The hue or colour temperature of the single light perceived by anobserver may be varied by modifying the relative proportions (i.e. arealdensity) of the different elements. Accordingly, the different arraysmay cover different proportions of the substrate, either by varyingsizes or numbers of elements. For example, the proportions of thesubstrate covered by the red, green and blue light-emitting material maybe present respectively in the ratio 100:1:3.7(?)

[0014] The light emitting elements may be mounted on a substrate andconfined within an area of 2.5×10⁹ m² (a square of sides 50 μm). Such alight emitting device would be on the same size scale as a lightemitting diode, and may thus be regarded as a point source emitter, atleast to the naked eye.

[0015] There is also provided an electronic device comprising an LCDdisplay and a backlight comprising a light-emitting device in accordancewith the first aspect of the invention.

[0016] In accordance with a second aspect of the present invention,there is provided a method of manufacturing a light-emitting device,comprising providing a first electroluminescent element for emittinglight of a first colour when energised; providing a second element foremitting light of a second colour when energised; characterised by:selecting dimensions of the elements and any spacing therebetween tocreate an overall impression of a single light source whensimultaneously energised and viewed as intended.

[0017] The method may further comprise energising the elements with acommon bias. This may be achieved by coupling the elements to a commonanode and a common cathode.

[0018] The method may further comprise providing a plurality ofelectroluminescent elements for emitting light of the first colour whenenergised, the plurality of first-colour light-emitting elements beingarranged in a spaced-apart array with the first electroluminescentelement. The method may also comprise providing a plurality ofelectroluminescent elements for emitting light of the second colour whenenergised, the plurality of second-colour-light-emitting elements beingarranged in a spaced-apart array with the second electroluminescentelement. The spaced-apart arrays of the first and second elements mayshare energising electrodes. In one embodiment, the method comprisesdisposing the first electroluminescent element adjacentsecond-colour-light-emitting elements.

[0019] The first or second organic light-emissive materials may bedeposited by a process of ink-jet printing.

[0020] In another aspect of the invention, there is provided alight-emitting device comprising a first element for emitting light of afirst colour, a second element for emitting light of a second colour,and an electrode common to both elements for actuating light emission,wherein the size and any spacings between the first and second elementsare selected to create the impression of a single light source when theelements are actuated simultaneously and viewed as intended.

[0021] The first and second elements may comprise semiconducting orelectroluminescent materials. The light-emitting device may furthercomprise an additional electrode common to both elements for actuatinglight emission.

[0022] An embodiment of the invention will now be described, by way ofexample, with reference to the accompanying figures, in which:

[0023]FIG. 1 is a schematic illustration of an organic light-emissivedevice known in the art;

[0024]FIG. 2 is a schematic underside view of part of anelectroluminescent device embodying the present invention;

[0025]FIG. 3 is an intensity/wavelength plot for light which may beemitted from a device of the kind shown in FIG. 2;

[0026]FIG. 4 is a schematic cross-sectional illustration of a displaycomprising the electroluminescent device of FIG. 2; and

[0027]FIG. 5 is a flow chart illustrating the fabrication of the deviceof FIG. 2.

[0028]FIG. 2 illustrates part of an electroluminescent device (10) foruse in a small electronic article embodying the present invention, asviewed through a transparent glass substrate (12) and a transparentfirst electrode (13). On the underside of the substrate (12) and firstelectrode (13), three arrays of different organic light-emissivematerials are deposited, each of which is able to emit light of adifferent colour when energised, namely red (R), green (G) and blue (B).Each array comprises a multitude of dot-like elements (14), withelements of different arrays being interleaved without overlappingadjacent elements to provide a relatively homogeneous distribution ofelements across the surface of the substrate (12) and first electrode(13). The diameters (D₁,D₂,D₃) of the dot-like elements (14) varyaccording to which organic light emissive materials are used, but allare typically 5×10⁻⁶ m or less. Similarly, the spacings (S_(x) andS_(y)) between the dot-like elements (14) in the X and Y directionsvary, but all are typically 5×10⁻⁶ m or less. Neither the individualdot-like elements (14) nor the spacings between them are capable ofbeing resolved by the naked eye of an observer. The relative proportionsof dot-like elements (14R:14G:14B) are chosen to provide particularamounts of red, green and blue light which give the impression of awhite-light-emitting source.

[0029] Suppose, for example, that the three different organiclight-emissive materials emit light with the following CIE co-ordinates:(0.677,0.311) Red; (0.400, 0.573) Green; and (0.178,0.220) Blue. Giventhe luminance voltage/current characteristics for light emitting polymerdevices (with electroluminescent layer thicknesses in the range 60-80nm), the relative area of each material (scaled to the green emitter)required to produce one kind of white light is shown in Table 1 forvarious voltages. The corresponding luminance of the materials is shownin Table 2 for various voltages. It is noted that not only does theluminance of each material vary with voltage, but also the luminance ofthe three materials at the same voltage is significantly different. Atlow voltages and low luminance requirements (say 3-5V and circa 100cd/m²), the thickness of the electroluminescent layers producing the redand green light may be increased to even up the area ratio.

[0030] According to Table 1 at an operating bias of 5 volts, therequired areal ratio of blue to green is 8 to 1, and the areal ratio ofred to green is approximately 2 to 1, in order to produce one kind ofwhite light, e.g. white light of a cool colour, corresponding to say600K. By varying the ratios of blue to green and red to green, differentwhite-colour hues may be achieved. It is possible to produce exampleswhich achieve the following white colour points: (0.374,0.330) and 3V;(0.355,0.375) at 4V; and (0.355,0.376) at 5V. Two typical white spectraare shown in FIG. 3; one curve has been calculated to give the whitecolour (0.33,0.33) at 4V and requires the following ratio (100:1:3.7) ofred-light: green light: blue light producing areas.

[0031] The display (30) of FIG. 4 comprises the electroluminescentdevice (10) of FIG. 2 used as a backlight underneath a planar LCD unit(20), such that light emitted from the device (10) can pass through anylight-transmissive regions in the LCD (20) and towards an observer (22).The light-emissive regions of the device (10) are sandwiched betweenanode and cathode electrodes. The cathode electrode (24) is common toall light-emissive regions, as is the anode electrode (13) Thus thelight-missive regions (14R,14G,14B) are energised by a common bias. Theanode (13) is formed of a light-transmissive material which is depositedon the glass substrate (12).

[0032] The LCD unit (20) is a normal passive-matrix LCD device andincludes a pixel (40) which may be activated using electrodes (42,44) tocontrol transmission of light from device (10) through the pixel (40).When light is transmitted through or around the pixel (40), the observer(22) perceives the backlight as a white emitter. This is because thenaked eye of the observer (22) is unable to resolve the individualelements (14R,14G,14G) and spaces therebetween and thus the lightemitted by each element is summed by the eye to give the impression of asingle source of white light.

[0033] Referring to FIG. 5, a method of fabricating the light-emittingdevice (10) will now be described. A transparent substrate (12) isprovided at step (50) and is then coated with a transparent firstelectrode (13) at step (52). Next, at step (54), red, green and bluelight emissive materials in liquid form are deposited by a process ofink jet printing dot-like elements onto the transparent first electrode.The size of the dot-like elements may vary between the differentmaterials in order to achieve the correct proportions required forproducing white-light emissions. Alternatively, or additionally, therelative numbers of dot-like elements may vary in order to achieve thedesired proportionality. After the deposited materials have dried orbeen cured, the dot-like elements are electrically coupled at step 56 toa planar second electrode which may have a profile including recesseswhich register with gaps between the dot-like elements. The recesses mayhelp to reduce short circuits between the first and second electrodes.The application of a common electrical bias of say 5 volts across thefirst and second electrodes will give rise to electroluminescentemissions from the light-emissive materials in an amount to yield acummulative white-light effect to an observer. TABLE 1 Relative areas ofred, green and blue light- producing materials which are required forone kind of white light, at given voltages. 3 V 4 V 5 V Blue 100 50 8Green 1 1 1 Red 0.98 1.9 1.9

[0034] TABLE 2 Luminance (cd/m²) of red, green and blue light-producingmaterials at given voltages. 3 V 4 V 5 V Blue 2.7 232 1260 Green 5615756 11591 Red 147 475 944

1. A light-emitting device comprising a first electroluminescent elementfor emitting light of a first colour when energised and a secondelectroluminescent element for emitting light of a second colour whenenergised, characterised in that dimensions of the first and secondelements and any spacing therebetween are selected to create an overallimpression of a single light source when simultaneously energised andviewed as intended.
 2. A light-emitting device according to claim 1,further comprising means, common to both elements, for energising theelements.
 3. A light-emitting device according to claim 1 or 2, in whichat least one of the electroluminescent elements comprises an organiclight-emissive material.
 4. A light-emitting device according to claim1, 2 or 3, in which the first electroluminescent element is one of aplurality of such elements for emitting light of the first colour whenenergised, the plurality of first-light-emitting elements being arrangedin a first spaced-apart array.
 5. A light emitting device according toany one of claims 1 to 4, in which the second electroluminescent elementis one of a plurality of such elements for emitting light of the secondcolour when energised, the plurality of second-light-emitting elementsbeing arranged in a second spaced-apart array.
 6. A light emittingdevice according to claim 5 when appendent to claim 4, in which eachelement of the first spaced-apart array is adjacent to, or disposedbetween elements of the second spaced-apart array.
 7. A light emittingdevice according to any one of claims 1 to 6, in which dimensions of theelements and any spacings between adjacent elements are unresolvable tothe naked eye.
 8. A light emitting device according to any one of claims1 to 7, in which the elements are disposed on a substrate.
 9. A lightemitting device according to claim 8 in which maximum dimensions of theelements over the surface of the substrate is less than 5.0×10⁻⁶ m. 10.A light emitting device according to claim 8 or 9, in which the maximumspacing between adjacent elements over the surface of the substrate isless than 5.0×10⁻⁶ m.
 11. A light emitting device according to any oneof claims 1 to 10, in which the first and second colours of lightemitted by the first and second elements are selected from the groupconsisting of red, green and blue.
 12. A light emitting device accordingto any one of claims 1 to 11, further comprising a thirdelectroluminescent element for emitting light of a third colour whenenergised, the dimensions of the third element and any spacing betweenthe third and adjacent elements being selected to create the impressionof a single light source when all the elements are energised and viewedas intended.
 13. A light emitting device according to claim 12, in whichthe first, second and third electroluminescent elements are selected toproduce red, green and blue light emissions respectively.
 14. A lightemitting device according to any one of the preceding claims, in whichthe single light source is perceived to produce substantially whitelight.
 15. A light emitting device according to claim 14, in which thesubstantially white light has a hue dependent upon the relativeproportions of the elements.
 16. A light emitting device according toany one of claims 3 to 15, in which the organic light-emissive materialsin the at least one element is deposited by a process of ink-jetdeposition.
 17. A light emitting device according to any one of claims 1to 16, in which the first and second elements are energised by a pair ofcommon electrodes.
 18. An electronic device comprising an LCD displayand a backlight, the backlight comprising a light-emitting deviceaccording to any one of claims 1 to
 17. 19. A method of manufacturing alight-emitting device, comprising: providing a first electroluminescentelement for emitting light of a first colour when energised; providing asecond electroluminescent element for emitting light of a second colourwhen energised; characterised by: selecting dimensions of the first andsecond elements and any spacing therebetween to create an overallimpression of a single light source when simultaneously energised andviewed as intended.
 20. A method according to claim 19, furthercomprising providing means, common to both elements, for energising theelements.
 21. A method according to claim 19 or 20, further comprisingproviding a plurality of electroluminescent elements for emitting lightof the first colour when energised, the plurality of first-colour-lightemitting elements being arranged in a spaced-apart array with the firstelectroluminescent element.
 22. A method according to claim 19, 20 or21, further comprising providing a plurality of electroluminescentelements for emitting light of the second colour when energised, theplurality of second-colour-light emitting elements being arranged in aspaced-apart array with the second electroluminescent element.
 23. Amethod according to claim 22 when appendent to claim 21, in which thefirst electroluminescent element is adjacent or disposed betweensecond-colour-light emitting elements.
 24. A method according to any oneof claims 19 to 23, in which at least one of the electroluminescentelements comprises an organic light-emissive material.
 25. A methodaccording to claim 24, further comprising depositing the organiclight-emissive material by a process of ink-jet deposition.
 26. A methodaccording to claim 25, in which the deposited organic light-emissivematerial comprises at least one dot having a maximum dimension of5.0×10⁻⁶ m.
 27. A light emitting device substantially as hereinbeforedescribed with reference to and as illustrated in the accompanying FIGS.2-5.
 28. A method of manufacturing a light-emitting device substantiallyas hereinbefore described with reference to and as illustrated in theaccompanying FIGS. 2-5.